Those Linked Articles
Lee: thanks for posting those links. The first claims that working in a convenience store is the second most dangerous occupation, behind taxi cab driving. Interesting.
The first also cites multiple conflicting studies, accedes to ambiguity, and relies in at least one case upon a single study to support a curious claim. Somewhat the same story in the other ones.
Nonetheless, they seem generally accurate in their descriptions of how such crimes occur, and who perpetrates them, so I'll reach into my experiential bag to provide a firsthand sample of the problem.
The Landscape
If one considers taking action in such an environment, what may he encounter? I'll start here...
contributor
By the way, has it occurred to anyone that, if the robber is not sufficiently aware of what you are doing to defend himself against you, that most probably means that his accomplice is?
My generalized experience with such fellows is that they are focused on their extremely limited plan, and that they always follow the same ones, whether the criminal is alone or with another. There are two plans. Here's the first one: Get close. Spring surprise. Take money. Leave. Here's the other one: Act immediately. Overwhelm with swift action. Take money. Leave.
My particular experience with such fellows is this: Roughly half of those making threats were alone, roughly the other half showed up in pairs. No one among threat-makers stood more than a dozen feet away. No one making threats moved when they made them. In the majority of cases, their game was first to gauge the clerk's reaction and to then act, depending upon reaction. The accomplice, when there was one, always looked where the primary actor looked when the threat was made. (That means they were looking at the clerk.)
As mentioned above, no one making threats stood more than a dozen feet away. But where were other customers, and what were potential distances involved?
Potential Distances
The store where I worked was of typical square footage and arrangement (at least for early eighties vintage shops), with the longest potential in-store distance at around forty-nine feet (diagonal measurement across interior). N/S distance was about 34 feet, E/W distance about 35 feet. Add another 4 to 6 feet to the N/S distance if you want to include behind-the-counter space.
“Waiting area” put customers in standing-room proximity. In other words, if you were in line after shopping, you would have been no more than a dozen feet away from the theoretical BG, most likely to his left, with something in your hands, less likely behind him. If you were not in line, the greatest
likely in-store distance would be right around 42 feet. This is based upon the typical location of threat makers (near the front counter) and the typical greatest distance from it occupied by customers. (In other words, that's how far away the beer was from the front counter, beer being a popular night-time draw.)
Most of the time, however, the greatest typical distance did not apply. Common separations were about twenty feet, and by this I mean the distance between a customer at the register (or theoretical BG) and a customer still shopping.
Most Likely Distance
So what does this mean? In this environment (a pretty typical one, as c-stores go), a shoot-inclined customer could find himself facing a distance somewhere within forty-eight feet, but most likely within the statistical 21 feet or less.
If there were two BGs, how close together were they at time of threat? No more than five feet apart. Most often, in fact, they were closer.*
Could one sneak up on the theoretical BGs in this environment? Maybe. As mentioned, once threats were made, the BG or BGs did not not look elsewhere.
Summation
I point out all of the above only to say this: if you find yourself in such a situation, it's probably going to be at close range (21 feet or less), and it's very likely to be at very close range (right around a dozen feet).
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*NOTE: I'm ignoring instances in which the store was cased first, since the OP's scenario involves the moment of announced robbery.